Wireless communication is used as a technique of avoiding loads of wiring works in conventional wired communication and realizing mobile communication. For example, examples of a general standard of a wireless LAN (Local Area Network) include the IEEE (The Institute of Electrical and Electronics Engineers) 802.11. The IEEE802.111/g has been broadly used.
In many wireless LAN systems including the IEEE802.11, an access control procedure based on carrier sense such as CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) or the like is employed so that communication stations avoid collision of carriers at a time of random channel access. Specifically, a communication station which corresponds to generation of a transmission request monitors a medium state only for a predetermined frame interval DIFS (Distributed Inter Frame Space). When a transmission signal is not detected in the predetermined frame interval DIFS, random back-off is performed. Furthermore, when a transmission signal is not further detected while the random back-off is performed, the communication station obtains an exclusive-channel use/transmission right (TXOP: Transmission Opportunity) of an exclusive channel and is allowed to transmit a frame. Furthermore, examples of a methodology for addressing hidden terminal problem in a wireless communication include “virtual carrier sense”. Specifically, the communication station assumes, when information on a duration (sustained period) for reservation of a medium is written in a reception frame to be received by another communication state, that the medium has been used in a period of time corresponding to the duration information, that is, performs carrier sense and sets a transmission stop period (NAV: Network Allocation Vector). By this, exclusive use of a channel in the TXOP is ensured.
The IEEE802.11a/g standard supports a modulation method which attains a communication speed of 54 Mbps at maximum (in a physical layer data rate) and which utilizes orthogonal frequency division multiplexing (OFDM) in a frequency band of 2.4 GHz or a frequency band of 5 GHz. Furthermore, a further high bit rate is realized in the IEEE802.11n standard which is an expansion standard of the IEEE802.11a/g standard by employing an MIMO (multi-Input Multi-Output) communication method. Here, in the MIMO communication method, a transmitter and a receiver have a plurality of antenna elements and a spatial multiplexing stream is realized (which is a known method). Although high throughput (HT) of 100 Mbps or more is attained by the IEEE802.11n standard, there is a demand for further speeding-up since an amount of information on transmission content is increased.
For example, by increasing the number of streams subjected to spatial multiplexing along with the increased number of antennas of an MIMO communication apparatus, throughput in one-to-one communication may be improved while backward compatibility is maintained. However, in the future, in addition to a throughput for each user in communication, throughputs for all users should be improved.
An IEEE802.11ac working group aims plot of a wireless LAN standard of a data transmission speed of 1 Gbps or more using a frequency band equal to or smaller than 6 GHz. To attain this plot, a space division multiple access method, such as multiuser MIMO (MU-MIMO) or SDMA (Space Division Multiple Access), in which wireless resources on a space axis is shared by a plurality of users becomes a possible candidate.
At present, the space division multiple access is discussed as one of fundamental technologies of a next-generation cellular phone system based on TDMA (Time Division Multiple Access) such as PHS (Personal Handyphone System and LTE (Long Term Evolution).
Furthermore, a communication system obtained by combining two techniques, that is, carrier sense based on the conventional IEEE802.11 standard and space division multiple access using an adaptive array antenna with each other using an RTS packet, a CTS packet, and an ACK packet which have packet formats which maintain backward compatibility with the conventional IEEE802.11 standard has been proposed (refer to Patent Literature 1, for example).
However, in a wireless LAN field, although one-to-many communication described above has been attracting attention, such communication has been rarely applied. This may be because it is difficult to efficiently multiplex a plurality of users in packet communication.
Particularly, under an infrastructure mode in which a plurality of terminals (MT) are associated with a single access point (AP) and are connected to a network as members of a BSS (Basic Service Set), when the terminals simultaneously perform uplink (UL) access to the access point, a difference of delay times among the terminals causes a problem. This delay time difference is mainly caused by transmission delays of the terminals associated with relative positions of the terminals relative to the access point and errors unique to the terminals such as clock accuracy. For example, in a specification of the IEEE802.11, an error of ±900 nanoseconds is tolerated as accuracy of a frame interval of transmission/reception. The delay time differences among the terminals which are larger than a Guard interval length used in OFDM cause interference among users when the terminals simultaneously transmit frames to the access point. Therefore, an effect of improvement of throughput caused by the space division multiple access is not expected.
To address the problem of the delay time differences among the terminals, a wireless transmission/reception system in which an access point employs a technique of removing interference and an equalization technique has been proposed (refer to Patent Literature 2, for example). However, implement of this technique in communication equipment causes increase of circuit cost which is an adverse effect when the access point is to be designed and fabricated at low cost.
Furthermore, as another method for addressing the problem of the delay time differences among the terminals, control of transmission times of the terminals is taken as an example. Although the delay time differences among the terminals may be controlled in a wireless LAN system in which TDD (Time Division Duplex) frames are clearly divided into an uplink and a downlink and in a communication system based on a reserved TDMA (Time Division Multiple Access) in which a base station takes initiative and a slot is ensured (refer to Patent Literatures 2 and 3, for example), the correction of the delay time differences is not applicable to a wireless LAN system based on the CSMA.